Monoamine oxidase (MAO), an anzyme involved in biogenic amine metabolism, is isolated, purified, and characterized from rat liver and brain mitochondria, guinea pig liver, and human platelets. Purified preparations are studied for cofactor requirements, presence of isoenzymes, kinetics, molecular weight, and subunit structure. The role of lipids and metals in MAO function is determined. The localization of brain mitochondrial MAO is traced, and the behavioral and biochemical effects of an MAO inhibitor, pargyline, are studied in the rat.